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I am installing a spa panel on an exterior wall and want to use the knockout on the rear of the panel. What should I use to attach cable to the panel on the exterior wall? Is there a specific clamp for this type of install? I plan to seal the back of the panel to the siding when it is screwed to the wall.

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I'd use a rigid pipe nipple long enough to go though the wall and some conduit lock nuts. Put plastic bushings on both ends of the nipple. The cable needs to be protected coming down the inside wall into the nipple. Technically, the cable that goes outside needs to be UF so you might want to run the UF the entire way but it's more expensive so you could run NB and then splice to UF in a junction box. All said and done, might be easier to run EMT the entire way and use individual wires. Depending on the size, you could use aluminum.

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    Put a junction box on the inside and you can use THWN in the conduit to the box - switch to NM cable there if you are going to use NM cable, but that way the wire into the exterior box is technically correct and waterproof as it should be. Don't forget duct seal to reduce air leaks and condensation; not to mention bugs. I stick with EMT inside the building, but I'm not normal, apparently (it's not a code requirement where I live.)
    – Ecnerwal
    Commented Feb 23, 2022 at 22:07
  • @Ecnerwal Yes to duct seal... From Chicago so born and raised with EMT, Rigid.
    – JACK
    Commented Feb 23, 2022 at 22:22
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    Yeah, since it's a spa subpanel it has aluminum rated terminals. If the entire run could be made with aluminum wire, that would save some coin to be sure. Aluminum is fine at these large sizes. Commented Feb 24, 2022 at 0:20

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